"slim" is generally used in a positive and complimentary way
"skinny" is less complimentary, sometimes a slightly negative word
"thin" is almost always negative and implies the person being underweight or unhealthy
I think the word "thin" is just as nasty as the word "fat". When people tell me I'm thin, I get angry. No, I'm not thin! A concentration camp survivor is thin. Likewise, I am not skinny. I am a fit, in-shape, healthy man with a normal BMI for my height, therefore I am slim.
Slim usually is about the body. The person looks nice this way.
Skinny is a term for not wide but a narrow thing - or a person or animal with little fat. The person may not look good at this weight.
Thin is a way to describe the width of something that is not thick, or a term for someone who is skinny and we do not know if it is becoming. It is a generic way to say it.
Much depends on the manner and context of using the words. Slim is always polite, skinny is being crtical without regard for being offensive and thin is more neutral.
slim has a more positive connotation than thin.
thin can be used to describe an unhealthy look (if someone has lost weight due to illness, they look thin, or if on the verge of being emaciated, etc)
slim is used to describe when someone is thin and beautiful at the same time. ie skinny in a good way.
The difference really is whether you look healthy or not.
however, people use each interchangeably these days, so it really depends the way it's being said.
"Skinny" is usually used in more extreme cases. If someone is bony, I call them "skinny." Any higher amount of flesh will make me consider them "thin." Generally, "skinny" is not a compliment to most adults. If you want to compliment someone, call them "thin."
In other words:
when you look at a girl and can see collarbones and hipbones jutting out, you think "dang - she skinny!" - that's skinny.
when you look at a guy and it's apparent where his bones are but you can't actually *see* them, you think "he could use a cheeseburger" - that's thin.
when you look at a girl and can't really see bones but she's dimensionally petite and think "wish i was that small" - that's slim
Thank you to everybody who has helped answer my questions！